1. Field of the Invention
The present invention relates to biogenic amorphous silica isolated from a plant material containing silica such as rice hulls, rice straw and so forth.
2. Brief Description of the Prior Art
Silica (silicon dioxide) occurs in crystalline and amorphous forms. Large quantities of synthetic amorphous silica are produced as pyrogenic (fumed) silicas and wet process silicas (precipitated silicas and silica gels) which are used, notably, for reinforcing elastomers, for thickening resins, paints and toothpaste, and as free-flow additives. Synthetic amorphous silica may also be ingested as a minor constituent (<2%) in a variety of food products where it serves as an anti-caking agent. Amorphous silica is also used in some pharmaceutical preparations for the same purposes.
Amorphous silica is found in nature as biogenic silica and as silica glass of volcanic origin. One form of biogenic silica, originates from the skeletons of diatoms deposited on sea floors but it contains a small amount of cristobalite (a crystalline form of silica) and quartz (another crystalline form of silica). Most producers of food and pharmaceutical products require that the amorphous silica used as an anti-caking agent contain no detectable amount of crystalline silica.
It is known that fibers of amorphous silica are produced by a variety of plants, such as sugar cane and rice. Of known plant materials, however, the rice plant is perhaps unique because of the high concentrations of silica that it contains. Whereas, the mineral content of most plants is, for example, about 1-2%, the rice plant typically has a mineral content of about 11-23%. More importantly, about 75-95% of the mineral content of the rice plant is silica. Of the rice plant, rice straw contains about 11% silica and rice hulls typically contain about 15-23% silica.
Rice hulls are the natural sheaths that form on rice grains during their growth. They are removed during the refining of rice and are a waste or a low-value by-product of the rice milling industry. Rice straw consists of stem, leaf sheathes, leaf blades and the remains of the panicle after harvesting. Generally, the amount of rice straw obtained from rice plants is at least equal to the rough yield of rice harvested. Because of their high silica content, rice hulls and rice straw have little value as components of animal feeds. Also, because these materials have a relatively large amount of potassium that interacts with the silica at combustion temperatures to produce boiler slag and deposits, and have a large fraction of noncombustible ash, they are similarly viewed as being a poor fuel source. For these reasons, rice hulls are frequently deposited in landfills and rice straw is usually burned in the fields. Thus rice hulls and rice straw have little or no commercial value and have historically presented a disposal problem.
Recent U.S. legislation has prohibited the use of synthetics (e.g., amorphous silicas such as fumed silica) in products which are marketed as organic foods. Currently there is an exemption to this law for synthetic silica because there is no biogenic silica. The present invention provides a source of biogenic silica that can be certified as organic.